So, after a solid 3 1/2 hours, I made a beam rifle for my RX-78-2 Gundam cosplay, and I got it approved by the cosplay committee so I can have it at the con. Reason I had to do that is because they are very anal about weapon props, and my beam rifle isn't the spitting image of the beam rifle in the anime. It shares the same shape, but that's all. It's not detailed all that much, and I'm painting it in a gray primer and that's it.

So, my question is this. To kinda let people know that it's not even real, I was gonna write on paper "This is not a real weapon" Then hot glue the message across the entire length of the prop on both sides, would it actually kindly tell people it's not real?

Well right off the bat, people would know it's not a real weapon. But for those who couldn't tell the difference, then the simplest thing you can do is make the tip of the muzzle (tip of barrel) a bright orange or just an orange as soon on Airsoft Rifles/Guns.

I'm not sure what con you're taking this too, but some places will "peacebond" your gun when you get there. Essentially, they wrap a bright colored zip tie or something around part of the prop so that everyone knows 1) it's not real, and 2) it's been approved by the con staff.

Zenithvsp's suggestion would work as well. Honestly, if the con is already cool with it, I don't think you'll have much of a problem.